The subject matter disclosed herein relates to structured-light based measurement, and more particularly to a structured light based method for determining the distance from a probe of a video inspection device to an object (“object distance”).
Video inspection devices are used in a wide range of applications. In some applications, such as optical examinations of organs inside living bodies using an endoscope, or examinations of defects in commercial equipment, it is useful for the user of the video inspection device to be able to determine the probe's distance from the object examined in order to perform measurements on that object. To accomplish this and other viewing tasks, current probes employ a variety of methods. Some examples of measurement methods include stereo, shadow, and projected dot grid methods.
Stereoscopic systems generally use a special optic system to view the same scene from two vantage points, relying on a surface detail of the object in the images to match the two images. Object distance is determined by analyzing the slight differences in the images. Projected dot grid methods use a light source, such as a laser, to project dots onto the object. Spacing between the dots is then determined or the positions of the dots in the image are then determined in order to determine the object distance. Shadow methods place a single opaque element, such as a line, between a light source and an object. The element is positioned in the light emitted by the light source, offset at an angle from the centerline of the light and the light source. If the object is in the portion of the light field that contains the shadow cast by the opaque element, then as the object moves closer or farther from the apparatus, the position of the shadow in the image shifts and can thus be used to determine object distance.
Current measurement methods each have a variety of limitations. For instance, stereoscopic systems have a baseline spacing that is limited by the physical dimensions of the apparatus, including its bifocal viewing optics. The baseline spacing determines the resolution of the probe. Increasing the baseline spacing can provide better accuracy at a given object distance. Furthermore, in stereoscopic systems, the same point on the viewed object must be identified in both images in order to compute object distance. Many surfaces lack uniquely identifiable features, which makes the accurate determination of object distance difficult or impossible.
With shadow measurement methods, if the object is not in a portion of the field of view containing the shadow, no measurement can be taken. Furthermore, only one specific area is measured rather than a large field of view, so surface irregularity over the field of view and orientation of the object in the field of view are undetected.
In many stereoscopic measurement systems and shadow measurement systems, two sets of optics are used. A first set of optics is used to view an object, while a second set of optics is used to take measurements. The second set of optics, often contained in a separate probe tip, must be interchanged with the first set when a measurement is desired. For instance, in one shadow measurement system, the same general viewing light source is used for general viewing and for measurement. However, a separate shadow measurement tip must be installed to perform measurements with the general viewing light source when a defect or other measurable feature is discovered. This interchanging of probe tips consumes additional time and detracts from the efficient use of the probe. Furthermore, shadow measurement optics significantly block light output, so that while the shadow measurement optics are used, the field of view is less well illuminated, which limits the viewing distance. Stereoscopic optics are also undesirable for general viewing as the image resolution and viewing depth of field are general reduced relative to those of normal viewing optics.
In other instances, a human subjective component (e.g., estimating where a shadow or other pattern falls in an image provided by a display, etc.) is involved that limits accuracy and prevents automatic measurements. Also, many probes or probe head assemblies are large or bulky, often because of the complexity of the design and/or arrangement of the viewing optics. Smaller and/or simpler viewing optics enable smaller probes and/or probe tips, with greater ability to be manipulated in tight spaces, or greater room to design and/or incorporate additional functionality.
It would be advantageous to determine the distance to the surface of an object during inspection without the disadvantages of the above systems.